The US has ample satellite means of reconnaissance to identify the plane, if it ws at any known airport. An off-airport landing would likely trigger the ELT, unless it can be deactivated in flight. It can't on my plane.
I can easily arm or disarm my ELT from the left seat...
I can't envision the airplane getting into or beyond Pakistan or any of the other 'stans without U.S. surveillance assets seeing it. Same for the respective militaries of India and China and their territories. But that doesn't mean it's not there. They may have seen it but not recognized it for what it was, or they allowed it in with full knowledge of what was going on.
For one thing, it's not impossible to file and fly as a different aircraft, even one operated by a government, with full ATC knowledge. And it's unlikely any U.S. surveillance assets were looking in the right direction at the right time to pick out
this T7 from the clutter, but there may have been some non-scheduled flights acting like a big Boeing. I think theories about it tailgating another aircraft are a bit far-fetched because of the skills required, but masquerading as yet another Boeing doesn't seem all that difficult.
At the end of the day, if this isn't a result of a mechanical failure, a botched hijacking that ended over water, or suicide, it's a very sophisticated and well-executed plan, to an unknown end. If so, the airplane likely is on the ground somewhere, well-camouflaged or hangared, perhaps having landed before dawn at a secluded location, like a military base. That means state actor, or at least someone acting with a government's tacit approval and a sophisticated, well -planned, -disguised and -implemented endeavor. So far.
Who has the resources and testicular fortitude to plan and pull this off?
Ultimately, though, I think the highest probability here is Captain Speaking did a deadstick lawn dart into the IO, and some trace will be identified in 48 hours or so.